In general, a bearing is a core part in a rotor field, and oil or gas is injected therein to minimize mechanical friction during rotation.
In a conventional bearing, mechanical contacts inevitably occur, because a bearing shaft is rotated or a ball bearing is used and rotated. When such a bearing is used in a galvanizing line, the bearing may be gradually abraded while being rotated in a galvanizing bath in which the temperature ranges from 450° C. to 470° C. Furthermore, since the bearing may cause an unstable rotation, vibrations and noise may occur when a roll is rotated.
In particular, such vibrations may increase a coating weight variation, when a steel plate passes through an air knife which is a gas wiping device provided at the rear side thereof. Then, it may cause a defect in products.
Therefore, in order to prevent such a problem caused by the mechanical contact and abrasion, the roll shaft needs to be periodically replaced. For example, the roll shaft may be replaced every two or three weeks. However, as the equipment is increased in size and the line is increased in speed, the lifespan of the mechanical ball bearing may be rapidly shortened.
FIG. 1 illustrates a conventional hot-dip galvanizing device. In the conventional hot-dip galvanizing device illustrated in FIG. 1, a steel plate 6 passing through a galvanizing bath 1 passes through a sink roll 3, a stabilizing roll 5, and a correcting roll 4 and then passes through an air knife 2, an electromagnetic stabilizer 8, and a top roll 7. The temperature of the molten zinc in the galvanizing bath 1 ranges from 450 to 460° C., and steel plates 6 having a variety of widths and thicknesses may pass through the galvanizing bath.
A load applied to a roll shaft may differ depending on the steel plates 6 to be supplied. In the case of the sink roll 3, a maximum load of 500 kgf may be applied to both ends thereof. Furthermore, a load of ±100 kgf may act in the rotation direction of the roll due to a dynamic characteristic such as a vibration, and a maximum load of less than 100 kgf may act in the axial direction thereof.
In such a hot-dip galvanizing process, a constant force does not act at all times, because of various kinds of steels and tension. Therefore, when a high-strength steel or high-tensile steel is processed, a hot-dip galvanizing device is required, which exhibits a rapid response and a strong physical characteristic even though a force acting at both ends of a roll in a radial direction and a force acting in an axial direction are changed, and has a long lifespan because it may be used in a non-contact manner.
An object of the present invention is to provide a magnetic bearing device for supporting a roll shaft, in which the main factors having an effect upon the performance of an electromagnetic force are optimally designed depending on a result obtained by analyzing the characteristics of the magnetic bearing device.
Another object of the present invention is to provide a magnetic bearing device which controls a current applied for stable driving in order to remove vibrations in a roll and a steel plate.
Another object of the present invention is to provide a magnetic bearing device which reduces a coating weight variation when an air knife performs gas wiping and enables a roll shaft within a galvanizing bath to be used semi-permanently.
Another object of the present invention is to provide a magnetic bearing device which obtains a rapid response speed greater than that realized in the case that an existing proximity sensor is used, by using a magnetic bearing which does not used an expensive sensor.